Checking tefillin, and what if the tefillin or the straps were damaged
1. If tefillin are assumed to be technically valid according to the laws, they may be assumed to be valid provided their coverings are undamaged, and they do not require checking; we do not worry that perhaps a letter may have become erased inside, or that a hole may have formed. Nonetheless, it is proper to check them, as sometimes they become damaged from sweat. If one wears them only periodically [=not every day], they need only be checked twice in seven years; they should also be checked if the covering becomes damaged, or if they fell into water. But if there was no-one there who knew how to check them, put them back together and sew them up, he should leave them be without tefillin, so as not to be entirely unable to do the mitzvah [they might still be kosher if you leave them be, but if you don’t put them back together properly they’re certainly not kosher].
2. If the exterior of the housing became torn, whether shel rosh or shel yad, it is invalid (P.I. Oraḥ Ḥayyim siman 10), but – in the shel rosh only – if the exterior was sound and two adjacent partitions were torn it is invalid; if they were not adjacent, such as the first and third, they are valid, since there is an unbroken compartment between them.
3. If their squareness was damaged, they are invalid until fixed. One must be very, very careful about this.
4. If two adjacent stitches were broken, or three stitches even if they weren’t next to each other, they are ruled invalid from doubt. If one has no others, he may lay them even before they are repaired, but without a blessing.
5. If the portions became torn, the rule is as for a sefer Torah, in chapter 18.
6. If the straps became broken such that they were now shorter than the minimum length (chapter 23:4) knotting or sewing them together is of no avail. If such that they were still of the minimum length, they may be knotted or sewn. There are those who say that knotting and sewing are of no use for the parts of the strap which go round the head and arm but may be done on the parts of the strap which are past them. There are also those who say that they may even be sewn on those parts which go round the head and arm, just not knotted, provided it is sewing which is done on the inside and not on the outside. In pressing circumstances, one may rely on these leniencies so as not to be entirely unable to do the mitzvah, but it seems to me that one should not make a blessing if his straps are shorter than the length given in 23:4, even if they are sewn together. Further, one should be stringent and sew them, not tie them, and where possible he should sew them with gid. If one’s strap broke but was still long, and there remained only a very little less than the proper length, we can say that it is proper to sew them in any case, but this also requires investigation (Peri Megadim) (and see chapter 25:5; it is forbidden to reverse the points of the straps).